A rib-like projection is provided on a back surface of a thin wall portion of a resin-molded component to reinforce the thin wall portion or to perform positioning of the component when another component is attached thereto.
When injection-molding is performed, the rib-like projection disturbs a flow of resin injected from an injection gate into a cavity of a mold for forming the thin wall molded-component. This affects a front surface of the thin wall portion of the resin-molded component, which is a front surface of the rib-like projection, so that sometimes, color unevenness, luster unevenness, tarnish, a sink mark or the like occurs thereon.
Japanese Patent Application Laid-Open No. 2001-171455 discusses, as means for avoiding color unevenness, luster unevenness, tarnish, a sink mark and the like, a method for preventing the disturbance of a resin flow by cutting a rib corresponding to a part at which the disturbance of the resin flow occurs during molding. Japanese Patent Application Laid-Open No. 7-276445 discusses a method for providing a space between the intersection part of ribs and a surface on which the ribs are formed, at which the disturbance of resin is likely to occur. Japanese Patent Application Laid-Open No. 2009-29252 discusses a method of setting a thickness of a root portion of a rib to be larger than that of a distal end portion of the rib.
Japanese Patent Application Laid-Open No. 2009-45918 discusses a method for forming a resin-molded component using a mold in which a gas vent is provided in a cavity to form a rib-like projection, and setting a mold temperature to range from 60 degrees centigrade (° C.) to the injection temperature of resin, moreover the thickness of a thin wall portion is set at 2.7 times or more the thickness of the rib-like projection.
However, in recent years, there have been increasing demands in apparatuses for resin-molded components whose outer appearance surfaces are configured as specially designed glossy surfaces, mirror surfaces, and high-quality embossed surfaces, and demands in optical apparatuses for resin-molded components having high-functionality mirror surfaces which reflect images or the like.
However, the methods respectively discussed in Japanese Patent Application Laid-Open Nos. 2001-171455, 7-276445, and 2009-29252 cannot sufficiently obtain effects of suppressing the unevenness and the like on the specially designed outer appearance surface of the thin wall portion.
According to the method discussed in Japanese Patent Application Laid-Open Nos. 2009-45918, if the mold temperature is set higher, it is necessary to set a cooling time longer. Consequently, a molding cycle time becomes longer. If the thickness of the thin wall portion on which a luster surface is formed is set at 2.7 times or more the minimum necessary thickness of the rib, the thickness of the thin wall portion becomes thicker than necessary. Thus, increasing an amount of used resin causes the cost and the weight of a product to increase, and requires longer time for cooling the product. In addition, resin shrinkage continues even after the product is taken out of the mold, so that an amount of the shrinkage is increased. Thus, other problems occur, which are, e.g., change in the total length of the product and that in the amount of warpage thereof. On the other hand, if the thickness of the thin wall portion is set at the necessary minimum amount, it is necessary to reduce the thickness of the rib. Thus, the thickness necessary for the function of the rib cannot be assured. In this case, integral molding of the rib and the thin wall portion should be given up. A member corresponding to the rib should be attached to a molded body by bonding, welding or the like.
Patent Citation 1
    Japanese Patent Application Laid-Open No. 2001-171455Patent Citation 2    Japanese Patent Application Laid-Open No. 2009-29252Patent Citation 3    Japanese Patent Application Laid-Open No. 2009-45918